I'm buying Skyrim now, because I want to play it now. If expansions and such are released during my playtime with the game, and I find myself enjoying it enough then I'll buy them. It might be cheaper in the long run to just buy this GOTY, or Ultimate edition or whatever they call it at the time, but that usually adds another year of waiting at minimum, besides the DLC and expansions usually come out on rather spaced out dates and by the time their released I will have more money and thus my cash flow isn't generally affected by a slightly increased total price over waiting another year.
That's my logic on it at least.